Seat in the upper chamber (5)
I believe the answer is:
perch
'seat' is the definition.
(perch is a kind of seat)
'in the upper chamber' is the wordplay.
'in' indicates a hidden word.
'PERCH' can be found hidden inside 'upper chamber'.
(Other definitions for perch that I've seen before include "Bird's seat - fish" , "Fish and imperial measure" , "Freshwater game fish" , "Fish; pole" , "[SOMETHING FISHY]" .)